Before committing suicide on Sunday, Keyes confessed to at least seven other slayings, according to the FBI field office in Anchorage, Alaska, which on Monday asked for the public's help with tracing Keyes' travels over the years in the hopes of identifying any additional victims.

He crisscrossed the country, and authorities may never know how many he killed.

"Based upon investigation conducted following his arrest in March 2012, Israel Keyes is believed to have committed multiple kidnappings and murders across the country between 2001 and March 2012," the office said in a statement. "Keyes described significant planning and preparation for his murders, reflecting a meticulous and organized approach to his crimes."

Investigators are continuing to investigate those crimes, though they say Keyes confessed to killing at least seven other people besides the barista: two in Vermont, four in Washington state, and one somewhere on the East Coast, disposing of the body in New York.

Keyes did not know any of his victims, the FBI said, but looked for them at remote locations like parks, campgrounds and cemeteries.

He is said to have buried supplies he planned to use in future crimes, and investigators recovered two caches, one in Eagle River, Alaska, and one near Blake Falls Reservoir in New York. They contained weapons and items used to dispose of bodies, the FBI said.

He flew into Chicago, rented a car and drove across several states before arriving there, the FBI said. He is then thought to have traveled around the East Coast before returning to Chicago and then Alaska, where he had lived since 2007.

Prior to that, he lived in Washington state, where he confessed to killing four people, the FBI said. He is likewise alleged to have admitted to killing another person, somewhere on the East Coast, in 2009. The identities of those five victims were not released.

Finally, Keyes was accused and charged in the death of 18-year-old Samantha Koenig, the Alaskan barista. She was last seen on February 1 being led away by a man from the parking lot of the coffee stand where she worked. Her body was found in a lake north of Anchorage.

Authorities in Anchorage released a chilling account of the killing of Koenig in part to "help investigations of other murders committed by Israel Keyes."

Keyes had planned to commit a kidnapping at Koenig's coffee stand even though he did not know her, according to the joint statement by local and federal authorities in Alaska.

He approached near closing time on February 1, wearing a mask and ordered coffee. After Koenig made the coffee, Keyes brandished a gun forced himself into the coffee stand and bound the 18-year-old woman with zip ties. She tried to escape, was unsuccessful and he took her to his truck.

"Keyes drove around town, explaining to Samantha that this was a kidnapping for ransom," the statement said.

He killed her at his home, hid the body in a shed and then went on a cruise he had been planning.

He returned on February 17.

"He went into the shed and retrieved Samantha's body, taking steps to make it appear that she was still alive, and took a Polaroid picture of her tied up," the statement said.

He sent the picture to girl's family members using her cell phone. He demanded $30,000 be sent to an account associated with an ATM card he had stolen from her.

The young barista's family was able to raise the money from donations and sent it. Authorities arrested Keyes after he made withdrawals of that money.

But not until the alleged serial killer made withdrawals in Anchorage, Arizona, New Mexico, and Texas.